Many homes and other buildings rely upon furnaces to provide heat during cool and/or cold weather. Typically, a furnace employs a burner that burns a fuel such as natural gas, propane, oil or the like, and provides heated combustion gases to the interior of a heat exchanger. The combustion gases typically proceed through the heat exchanger, are collected by a collector box, and then are exhausted outside of the building via a vent or the like. In some cases, a combustion blower is provided to pull in combustion air into the burner, pull the combustion gases through the heat exchanger into the collector box, and to push the combustion gases out the vent. At the same time, a circulating blower typically forces return air from the building, and in some cases ventilation air from outside of the building, over or through the heat exchanger, thereby heating the air. The heated air is subsequently routed throughout the building via a duct system. A return duct is typically employed to return air from the building to the furnace to be re-heated and then re-circulated.
In order to provide improved energy efficiency and/or occupant comfort, some furnaces may be considered as having two or more stages, i.e., they can operate at two or more different burner firing rates, depending on how much heat is needed within the building. Some furnaces are known as modulating furnaces, because they can potentially operate at a number of different burner firing rates and/or across a range of burner firing rates. The burner firing rate of the furnace typically dictates the amount of gas and air that is required by the burner. The circulating blower may be regulated, in accordance with the burner firing rate, to maintain a desired discharge air temperature, i.e., the temperature of the heated air returning to the building. A need remains for improved methods of determining burner firing rates.